1. Field of the Invention
The present invention relates to the structure of a semiconductor device and to a method of forming the same. More particularly, the present invention is directed to a contact structure of a semiconductor device and to a method of forming the same.
2. Description of the Related Art
As semiconductor devices become more highly integrated, the space between word lines and the space between interconnections of the devices are becoming more and more narrow. Accordingly, when an interlayer dielectric is formed for isolation between the word lines and between the interconnections, a void may be formed in the interlayer dielectric between the word-lines and between the interconnections. Thus, in order to prevent the void from being formed in the interlayer dielectric, the interlayer dielectric is formed by a chemical vapor deposition (CVD) method, and then a reflow process is performed. When the reflow process is carried out with respect to the interlayer dielectric, a low temperature of 900° C. or less is required in order to prevent an excessive diffusion of impurities at an impurity diffusion region. Accordingly, a borophosphosilicate glass (BPSG) is used as the interlayer dielectric, because the BPSG can reflow at the low temperature of 900° C. or less.
FIGS. 1A and 1B show semiconductor contact structures and a conventional problem in the forming thereof.
Referring to FIGS. 1A and 1B, a lower layer 3 is stacked on a semiconductor substrate 1. A pad 5 of polysilicon doped with phosphorus is formed on the lower layer 3, and then a first interlayer dielectric (ILD) 7 of the BPSG is formed over the pad 5. The reflow process is performed with respect to the first ILD 7. The first ILD 7 is patterned to form a contact hole exposing an upper part of the pad 5. The contact hole is filled with the doped polysilicon and the resultant structure is planarized by a chemical mechanical polishing (CMP) process to form a contact plug 9. A nitride layer 11 is stacked on the planarized surface so as to cover the contact plug 9. Thereafter, a second ILD 13 is stacked on the resultant structure. The second ILD 13 is formed of BPSG. The reflow process is performed with respect to the second ILD 13. At this time, the first ILD 7 formed of the same BPSG also reflows. Thus, as illustrated in FIG. 1B, the contact plug 9 falls over due to a slight movement in the BSPG of the first ILD 7. As a consequence, the contact plug 9 may lift off of the pad 5, resulting in a contact failure between the contact plug 9 and the pad 5. This phenomenon may occur whenever the contact plug is positioned in an interlayer dielectric formed of BPSG and a subsequent process is performed at a temperature that is high enough to cause the BPSG to reflow.